This invention relates to electrical imaging technology, and more specifically to an apparatus and method for producing high resolution images, with accurate values of the electrical properties of the imaged object.
Since Roentgen discovered the ability of x-rays to produce a shadowgram of the interior of a sample, there has been a substantial interest in scientific and engineering circles in technologies that allow the imaging of the interior of a sample using quantities measured exterior to the sample. While these technologies are often applied in industry and commerce generally, one of the most active areas for the use of imaging technologies is in the field of medicine. The original shadowgram x-rays were enhanced substantially with the discovery and perfection of computerized axial tomography, which allows the recovery of not just a shadowgram, but detailed information about the interior structure of a sample from x-ray intensities measured on the outside. A similar intense interest developed when nuclear magnetic resonance measurements were extended to map the interior of a sample in what is now commonly called Magnetic Resonance Imaging (MRI). Also, the use of ultrasound to explore the interior of samples has been a technology that has received substantial interest.
For all of these technologies (x-ray tomography, MRI, and ultrasound), an accurate picture in terms of spatial resolution is produced. However, information about the character of various objects located in the interior of a sample is often very limited. For example, x-ray tomography measures only the intensity of absorption at a single x-ray frequency and is generally simply proportional to the density of the material of the sample. The only improvements to conventional x-ray tomography have been to use x-rays at different frequencies, which allow some information about both the density of objects in a sample (in terms of mass) as well as the electron density. Nonetheless, at best, only two new pieces of information are available from such measurements. MRI is a modality that is sensitive to other parameters. Primarily, MRIs measure the number of protons (usually associated with hydrogen atoms) at any given point in a sample. Some extra information can be obtained with a great deal of analysis and care by measuring the decay time for certain magnetic resonance properties, but the sensitivity is such that perhaps three parameters can be measured using this technique. A similar situation is obtained with ultrasound where there are yet some other complications due to the multiple reflections of the sound waves. While CAT scans and MRIs produce pictures that are somewhat familiar to even the untrained eye, ultrasound imaging requires a very skilled operator to perform the measurements and to interpret the results.
Because of the limitations of the existing imaging techniques, scientists and engineers have looked for other properties that might be exploited to produce an appropriate and improved image of the interior of an object. Techniques have been developed which measure the electrical properties of different materials located within an object. Such imaging has shown that substantial variations within a sample from one type of material to another may be detected (e.g., in a biological sample such as a human being, from one type of tissue to another) and provides a unique imaging modality that reveals information quite different from conventional imaging modalities.
Such electrical property imaging techniques are often referred to as xe2x80x9cimpedance tomography.xe2x80x9d Most conventional electrical property imaging techniques are based on the premises that: 1) electrodes, or sensors, should be attached directly to the sample to be measured (for medical applications, the sample is a human body), and 2) current is injected sequentially through each electrode into the sample and the subsequent voltages measured. Therefore, these conventional imaging techniques implement a xe2x80x9cconstant current/measured voltagexe2x80x9d scheme.
In a departure from such conventional electrical property imaging techniques, one of the present inventors arranged sensors in an array outside the object to be measured as disclosed in U.S. Pat. No. 4,493,039. Further, during imaging of a sample, the ac voltages were applied at a fixed amplitude while the current was measured. This approach was further improved as described in pending patent application WO 99/12470 by filling the space between the object and the sensor array with an impedance matching medium. In addition, techniques for computing the internal charge distribution based on the measured surface charges are described, referred to as the scale factor technique and the iterative technique. Both the iterative and scale factor technique require initial estimates of the geometry of internal structures derived from an associated imaging system such as an x-ray CT system. The iterative technique also requires an initial guess of the electrical properties of each region, then uses a forward calculation of the expected currents at the boundary to check the validity of the guess, iterating this process until the guess produces boundary currents close to the measured values. The scale factor technique creates a xe2x80x9clook upxe2x80x9d table or neural net algorithm that allows one to correlate electrical properties or the interior of the sample with externally measured parameters using a large data set of model calculations. Because of limitations of the model and the need to extrapolate results to keep the size of the data sets reasonable, the scale factor technique has limited accuracy, but it does not require prior knowledge of approximate sample electrical properties. In fact, the results of the scale factor computation may serve as an initial estimate for the iterative technique. Both techniques are computationally intensive.
The present invention solves the problems associated with prior electrical parameter imaging techniques by providing an apparatus and method that generates an accurate image of the electrical properties of an object. More particularly, a charge correlation matrix is stored and employed during image reconstruction to directly calculate the internal charge distribution in the object from acquired surface charge measurements made on the object. From the calculated internal charge distribution, images of internal electrical properties such as conductivity and dielectric constants may be produced.
A general object of this invention is to produce high resolution images of internal electrical properties without the need for a separate imaging system. Charge correlation matrices are pre-calculated and stored in the apparatus for use with selected sensor array geometries and with prescribed image sizes and resolutions. The calculation of the internal charge distribution is a straight forward multiplication of the acquired surface charge data by the appropriate, stored charge correlation matrix. Electrical properties images are easily produced from the resulting internal charge image.